Galaxy mergers can transform the type of the parent galaxy into another and, dur- ing their occurrence, drive phenomena that are extraordinary compared to the pro- cesses that take place in quiescent galaxies. Specifically, simulations and observa- tions show that they trigger star formation events, and young objects at least as massive as globular clusters can be formed. Among the merger environments, lumi- nous (LIRGs; Lbol ∼ LIR = L[8−1000μm] = 1011-1012 L⊙ ) and ultraluminous (ULIRGs; LIR = L[8−1000μm] = 1012-1013 L⊙) infrared galaxies show the most extreme cases of star formation.
It is not surprising that the studies carried out so far on (U)LIRGs have found young compact star forming regions. However, only very few studies have been carried out so far for these kind of systems. This thesis work is devoted to the analysis of compact star forming regions (knots) in a representative sample of 32 (U)LIRGs, the largest sample used for this kind of study in these systems. The project is based mainly on optical high angular resolution images taken with the ACS and WFPC2 cameras on board the HST telescope, data from a high spatial resolution simulation of a major galaxy encounter, and with the combination of optical integral field spectroscopy (IFS) taken with the INTEGRAL (WHT) and VIMOS (VLT) instruments. This is the first time that such combination of different types (photometric, spectroscopic and numerical) of a large amount of data, and such detailed study is performed on these systems. A few thousand knots –a factor of more than one order of magnitude higher than in previous studies– are identified and their photometric properties are characterized as a function of the infrared luminosity of the system and on the interaction phase. These properties are compared with those of compact objects identified in simulations of galaxy encounters. Finally, and with the additional use of IFS data, we search for suitable candidates to tidal dwarf galaxies, setting up constraints on the formation of these objects for the (U)LIRG class. The main findings and conclusions are summarized as follows:
- With a typical size of tens of pc, the knots are in general compact, most of them are likely to contain sub-structure and therefore to constitute complexes or aggregates of star clusters.
- Even though (U)LIRGs are known to have most of their star formation hidden by dust and re-emitted in the infrared, we have observed a fraction of 15% of blue, almost free of extinction and luminous knots, with masses similar to or even higher than the SSCs observed in other less luminous interacting systems.
- An extinction correction, characterized by an exponential probability density func- tion, has to be applied to the colors of the compact stellar regions identified in simulations of major mergers in order to reproduce the rather broad range of colors sampled in knots in (U)LIRGs.
- Knots in ULIRGs, with higher star formation rate per unit area and gas content than less luminous interacting galaxies, are intrinsically more luminous (likely most massive) due to size-of-sample effects.
- Knots in ULIRGs can have both sizes and masses characteristic of stellar complexes or clumps detected in galaxies at high redshifts (z 1), intrinsically more massive than stellar complexes in less luminous interacting galaxies.
- The aging of the knots rules the evolution of the color distribution during the interaction. Theoretical and observational evidence shows that, as a consequence of the interaction process, only the most massive knots remain when the system relaxes.
- The slope of the luminosity function (LF) of the knots, compatible with α ⋍ 2, is independent of the luminosity of the galaxy. There are, however, slight indications the it varies with the interaction phase, becoming steeper from early to late phases of interaction. Supported by the simulation of a major galaxy encounter, higher knot formation rates at early phases of the interaction with respect to late phases, would explain such evolution of the LF.
- Among extranuclear star-forming Hα clumps identified in 11 (U)LIRGs, with typ- ical size up to several hundreds of pc, we identify 9 as candidates to tidal dwarf galaxies. They fulfill certain criteria of mass, self-gravitation and stability. With a production rate of 0.1 candidates per (U)LIRG systems, only a few fraction (< 10 %) of the general dwarf satellite population could be of tidal origin.